The purpose of the present invention is an increase in the resolution of active detection systems whose transmissions may be either electromagnetic, such as radars, or acoustic, such as sonars.
In these systems which are intended either for target direction or for extraction and identification or classification of certain parameters of possible targets or for image forming, the mode used, for sonar in particular, to illuminate the angular field to be observed is that of a single, coherent transmission, which covers the whole angular field to be observed, of duration T and bandwidth b emitted round a carrier frequency f.sub.o, while for reception, a set of angular channels is formed in parallel to cover this field.
The angular resolution of these systems is virtually that of the reception antenna and it is known that its dimensions add a limiting factor to this resolution.
It is known to increase the spatial resolution of systems in the sonar field with a linear acoustic base which uses, for transmission, a so-called interferometric mode that consists in transmitting two signals at the same frequency simultaneously from two transducers at the ends of the base.
The result of this transmission is the formation in space of zones that are sounded and unsounded alternately and, when convoluted with preformed channels on reception, give directivity improved by a coefficient of 2, the existence of the unsounded zones being the disadvantage of the method.
This can be corrected by two separate methods. Either a second simultaneous transmission is used, which is offset angularly with respect to the preceding one and which doubles the sweep time or two simultaneous transmissions are used, which are separate in frequency, one having the characteristics of the first transmission above and the other those of the other transmission, but then the useful band is doubled.
Another example of improvement in the directivity can be found in U.S. Pat. No. 4,119,940--Keating, which also uses N frequencies for transmission. This invention, which separates well the signals produced at the different frequencies but causes them later to lose their identity related to their different frequencies and does not do any space-time processing on them, is applicable neither for base-object relative movements nor for non-planar base.